It is often necessary in medical engineering to determine or monitor the concentration of selected components of the breathing air. For example, the concentration of the anesthetic gases being used in the breathing air is often measured during each breath in case of anesthetized patients or the alcohol content in the breathing air is measured when testing automobile drivers. Spectrometric methods are used to determine the concentration of the respective components. As a rule, signals are recorded for this with discrete filters in wavelength bands in which the gases have absorptions, and compared with references. The concentration of the gas in the breathing air can thus be inferred from the change in the signals at these wavelengths, which are characteristic of a certain gas.
A number of devices, with which the concentrations of selected components of a fluid or of a gas, for example, breathing gas, can be determined by absorption measurements at selected wavelengths or even over a spectral range, are known from the state of the art. Such devices have a radiation source, which emits radiation over a selected, continuous spectral range. An optical bandpass filter, called filter below for short, which limits the spectrum of the radiation to a wavelength that is characteristic of a component of the gas, whose concentration shall be measured, is arranged between the radiation source and a measuring section arranged downstream, in which the radiation passes through the fluid. After the radiation has passed through the filter and the measuring section, its intensity is measured with a suitable detector. The concentration of the component in the fluid can now be inferred from the attenuation of the intensity compared to a reference measurement in a reference fluid, in which the concentration of the component is known.
If the concentration of a plurality of components is to be measured, a separate, suitable filter must be used for each component. To determine the concentration of the various components within a short time period, possibly during one breath as a so-called measurement resolved for individual breaths, the different filters are arranged on a so-called filter wheel. A filter wheel is a rotating disk, which moves the filters in a rapid sequence into the beam path between the radiation source and the detector. This has, however, the drawback that only a small number of components can be analyzed, because a separate filter must be present for each component, and the size of the filter wheels is limited. In addition, the use of mechanical components is generally disadvantageous, because these increase the amount of maintenance needed and are prone to error.
A full absorption spectrum can be recorded with the use of a tunable Fabry-Perot interferometer, as it is described in DE 10 2006 045 253 B3. A tunable Fabry-Perot interferometer is arranged for this instead of a filter in the beam path between the radiation source and the detector. A Fabry-Perot interferometer is a device that has two partially transparent mirrors arranged in parallel to one another, whose reflective coated mirror surfaces point towards each other. The distance between the mirror surfaces determines a narrow wavelength range, which is transmitted by the Fabry-Perot interferometer and for which the Fabry-Perot interferometer is permeable. The width of the wavelength range passed through by the Fabry-Perot interferometer is called spectral resolution and depends on the wavelength. The distance between the mirror surfaces can be varied in a tunable Fabry-Perot interferometer and the wavelength range that is transmitted can thus be displaced. Thus, a Fabry-Perot interferometer is a displaceable bandpass filter, whose width corresponds to the spectral resolution of the Fabry-Perot interferometer. If the absorption spectrum of a fluid is to be detected completely over a selected spectral range, the Fabry-Perot interferometer only needs to be tuned over the selected spectral range.
In case of suitable coating of the mirror surfaces, a Fabry-Perot interferometer may also be permeable for two different wavelength ranges at the same time. It is consequently possible to record the absorption spectra of the fluid in both wavelength ranges simultaneously. A device for recording an absorption spectrum of a fluid with the use of such a Fabry-Perot interferometer is known from DE 10 2009 011 421 B3.
To determine the concentration of the individual components of the fluid, the spectrum recorded by the detector must be compared with a reference measurement. Lock-in amplifiers are routinely used for this. A measured signal is multiplied in a lock-in amplifier by a reference signal, and the result of the multiplication is integrated in a low pass filter. The lock-in amplifier consequently forms the cross correlation between the measured signal and the reference signal.
To make it possible to use a lock-in amplifier, the signal sent by the detector and hence the radiation emitted by the radiation source must, however, be modulated over time. Modulation over time shall be defined here as a change in the intensity of the radiation over time. This can be carried out in the simplest case by switching the radiation source on and off or by a chopper arranged downstream, which repeatedly interrupts the beam path between the radiation source and the detector.
The absorption of selected components in the breathing air is preferably measured at wavelengths between 2 μm and 15 μm. The broad-band radiation sources available in this wavelength range are usually thermal radiators. If these are modulated electrically over time with a frequency of more than 100 Hz that is necessary for measurements resolved for individual breaths, the relative modulation of the intensity depends strongly on the wavelength being considered, and the modulation decreases from short wavelengths to long ones. The intensity modulation decreases so strongly at longer wavelengths in the spectral range between 2 μm and 15 μm at frequencies of 100 Hz and higher that it is no longer sufficient for use with a lock-in amplifier.
Even though the use of a mechanical chopper is possible, in principle, this is again a mechanical component, which requires great maintenance efforts and is, moreover, difficult to miniaturize.
A similar problem arises when a photoacoustic or pyroelectric sensor is used as the detector, because these two types of sensors can only be used if the radiation has a change in intensity over time.